Eye Care Blog

When light rays from a distant object pass through the cornea only the central rays are likely to form an image on the central and most sensitive part of the retina. Only the central part of the cornea (an inner diameter of between 3 and 5 mm) is sufficiently curved to bend the light-rays regularly. The light entering the more peripheral parts of the cornea only stimulate the more peripheral parts of the retina. These are bent irregularly and do not form a clear central retinal image. This ‘peripheral vision‘ is most useful for locating objects in space, and, by a reflex nerve stimulation, regulating the size of the pupil.

When the eye is in a relaxed state and a distant object forms an image in front of (and not on) the retina, this is a state of myopia, or short-sightedness. The image quite literally falls short of the screen-receptor. If the distance between the object and the eye is decreased the position of the image in the eye will not change until that distance is less than six metres. The reasons for this are not too difficult to understand. Any light coming from a distance sends out light energy (particles) in all directions. Therefore, as they speed away from an object, they become increasingly divergent. But the eye’s opening (pupil) is only 2-10 mm in diameter. Thus an object has to be reasonably close to the eye for its emitted light rays to be more or less parallel when they reach the eye, and this distance, because of the fixed dimensions involved, happens to be at around six metres. This can be shown by drawing lines between the eye and the object concerned. Of course, light rays coming from an object more than six metres away will be ‘even more’ parallel; and within six metres the light rays will be more and more divergent. The normal eye can bend parallel rays to form a clear image on the retina. As the object draws closer, and its rays becomes increasingly divergent, the eye changes its focus and bends the light more and more.

A myopic or short-sighted eye, however, has difficulty in reducing its power when focused on far-off objects. It is not so much a question of weakness as of too much (optical) strength, relative to the size of the eye. The image of a distant object falls short of the retina. But as the object comes closer to the eye and the divergence of light increases there will come a moment when even in its relaxed state the natural power of the eye will compensate for the divergence and the image will fall on the retina. By establishing what this distance is one measures accurately the degree of myopia. It is the furthest point at which the myopic eye can see distinctly without the aid of an optical device such as spectacles. The less this distance is, the higher the degree of short-sightedness. Thus if you can only see detail at a distance of one metre you are a one-metre myope; and if the distance is 50 cm, you are a 50 cm myope. However, opticians, optometrists and ophthalmologists* do not use quite this terminology. The professional and technical term employed is a `dioptre‘. A dioptre is the reciprocal of a measurement in metre units. One dioptre corresponds to one metre, but two dioptres is one half of one metre, while half a dioptre is two metres. Thus if your natural focal length is one metre you are said to have one dioptre of myopia, while if you can focus on something 50 cm away you have two dioptres. ‘Perfect vision‘ means being able to focus in detail on something six metres away. Nowadays these metric measurements are standard throughout the world, but in the old days, when the metric system was not popular in English-speaking countries, measurements were made in feet and inches. Hence the expression ‘twenty-twenty vision‘, which meant that each eye could see in detail at a distance of 20 feet. Such good sight — or even better — is the good fortune of 75 per cent of the population.

There are very high degrees of myopia where the far point of good vision is only 5 cm from the cornea (20 dioptres). If the object is brought still closer to the eye it may still be seen in focus (`accommodation’); but in fact very highly myopic eyes do not generally have such powers.

The early, or young, short-sighted person, is difficult to detect since, under one dioptre of myopia, objects at a distance of between one and six metres away are subject to blurring. A slight contraction of the lids (`screwing up’ one’s eyes) will reduce the optical aperture of the eye (irrespective of the actual size of the pupil), and correct the focus. Indeed there are many people with a low degree of myopia who manage well enough without spectacles and do not even realize that they are shortsighted until they have their eyes tested. Such small degrees of myopia can only be corrected with spectacles, as the margin of error in the manufacture of contact lenses does not permit very small powers of corrective refraction. But low-power spectacles can be prescribed, and are often used by individuals with a small degree of myopia when, for example, driving a car or visiting the cinema.

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